Water from a borehole drilled by KCM at Shimulala village, Zambia. Photograph: John Vidal for the Observer

A London-listed mining giant has been polluting the drinking water of villages in Zambia and threatening a wider health disaster, the Observer has found.

Leaked documents and a confidential internal report commissioned from Canadian pollution control experts show that Vedanta Resources’ giant mine in Zambia’s Copperbelt region has been spilling sulphuric acid and other toxic chemicals into rivers, streams and underground aquifers used for drinking water near the mining town of Chingola.

‘I drank the water and ate the fish. We all did. The acid has damaged me permanently’

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The result, say people in four villages living near the giant 12 sq mile mine owned by Vedanta subsidiary KCM, is stomach pains and illnesses, devastated crops, loss of earnings and permanent injuries. The claims of villagers living near one of the largest copper mines in Africa are backed by a leaked letter from a KCM doctor stating that water collected for testing from Shimulala village in 2011 was unfit for human consumption. “The water is acidic and the copper and iron levels exceed permitted levels,” the doctor wrote. “The impurities … can cause cancer in the bloodstream and unhealthy conditions in internal organs. The people in that village should be advised to stop using the same water.”

London law firm Leigh Day has issued proceedings in the high court in London on behalf of 1,800 people who claim to have been affected by the company’s pollution. “The case could take three years to resolve,” said Leigh Day senior partner Martyn Day, recently returned from Zambia, where lawyers and paralegals have been taking witness statements from people living near the rivers and the company’s operations.

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A Vedanta spokesman said: “All Vedanta’s operating subsidiaries take the health of their employees, the wellbeing of surrounding communities and the environment very seriously. Our subsidiaries are committed to ensuring they operate in a safe and sustainable way.”

But a scientist who worked for more than 15 years with KCM said there has been little maintenance of critical equipment since Vedanta bought the mine, despite production of some 10,000 tonnes of copper and 300 tonnes of cobalt a year. He accused Vedanta of releasing more acid than it has authority for. “There have been heavy spillages and massive leakages. Acid has been leaking all over the place. The pollution control pond is handling too much material. No effort has been made to correct this scenario. Only one of four [waste] pipelines is running – the rest are in disrepair.

“Degraded equipment, leaking pumps, pipes, thickeners and settling ponds have [resulted in] excessive spillages. Water overflowing into the Mushushima river and subsequently the Kafue river poses a possible environmental catastrophe downstream,” he said.

“The company has very good plans on paper that have not materialised on the ground for the last 10 years. It is absolutely clear that there is a massive problem. Because the river Kafue feeds into the Zambezi river, which provides drinking water for much of Zambia, the pollution could affect hundreds of thousands of people downstream, he said. “A disaster is very likely. It has the potential of affecting people hundreds of miles away. Water supplies could be damaged and aquatic life would die.”

A leaked report by the Canadian engineering company SNC-Lavalin, which in 2010 was employed to advise Vedanta/ KCM on how to control continuing pollution, says that solids, dissolved copper and acids are being spilled. It refers to “constant contamination” of streams, and says the main pollution control dam is often full to capacity. It adds that reservoirs overflow and there are leakages from pipes and a lack of spare parts. The engineers’ report calls for 17 major and minor actions to stop the spillage of polluted water into the environment.

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In the villages near Africa’s biggest copper mine, you can smell and taste the pollution. As a legal battle against metals giant Vedanta/KCM reaches London, villagers in Chingola, Zambia, tell of blighted lives and a looming catastrophe